This invention relates in general to vehicle brake systems and in particular to a pedal simulator for a vehicle brake-by-wire system.
Most vehicles are equipped with a brake system for retarding or stopping movement of the vehicle in a controlled manner. A typical brake system for an automobile or light truck includes a disc brake assembly for each of the front wheels and either a drum brake assembly or a disc brake assembly for each of the rear wheels. The brake assemblies are actuated by hydraulic, pneumatic, or electrical mechanisms when an operator of the vehicle depresses a brake pedal. The structures of these drum brake assemblies and disc brake assemblies, as well as the actuators therefor, are well known in the art.
Typical conventional light vehicle brake systems are hydraulic, by which is meant that the brake pedal actuates a master cylinder, which in turn provides pressurized hydraulic fluid to actuate the brake assemblies. A hydraulic or vacuum booster may be provided to assist the brake pedal in actuating the master cylinder, but the braking demanded by the driver is conveyed to actuate the brake assemblies as mechanical forces and pressures.
In contrast, some brake systems are known in the art generally as brake-by-wire systems, by which is meant that at some point between the brake pedal and the brake assemblies at the wheels of the vehicle, the brake demand is conveyed as an electrical brake demand signal. This brake demand signal is used to control the operation of an electrical actuator. The electrical actuator may directly move a mechanism to force the brake pads or brake shoes of the brake assemblies into contact with the corresponding rotating brake surface, or may operate a hydraulic pump to pump brake fluid to actuate the brake assemblies. This latter type of brake-by-wire system, in which the brake demand is transmitted electrically to control a hydraulic pump, is also variously known as electro-hydraulic braking, electro-hydraulic control, or electronic brake management.
Typically, brake-by-wire systems are designed to make the driver interface with the brake system (the brake pedal) act similar to the brake pedal of a conventional hydraulic brake system in order to improve acceptance of brake by wire systems by consumers, and to avoid problems caused by drivers learning (or failing to learn) a new response during a braking situation. Thus, designers of brake-by-wire systems typically try to design a system in which the brake pedal will be easy to depress initially, but resist additional incremental depression of the brake pedal with ever increasing force, similar to a conventional hydraulic brake system.
One way that has been suggested to accomplish this mimicry of a conventional hydraulic brake system in a brake-by-wire system is to connect the brake pedal to a pedal travel simulator (referred herein as a pedal simulator) containing a spring which permits the brake pedal to be depressed against increasing force as the pedal is further depressed. Thus the pedal simulator enables the brake pedal to operate as if the pedal were connected to a conventional hydraulic brake system. The brake pedal may be directly connected to the spring by linkages, or, as is frequently the case in electro-hydraulic brake systems, the brake pedal can operate a master cylinder, the output of which is normally hydraulically directed to the pedal simulator. The brake demand signal is electrically generated as a function of the force exerted by the driver in moving the brake pedal to compress the spring, or as a function of the distance the brake pedal is depressed measured by a sensor, or as a function of both the force acting on the brake pedal and the distance the brake pedal is depressed.